FIG. 1 depicts a schematic diagram of an exemplary wireless local-area network (LAN) 100 in the prior art comprising stations 101-1 through 101-N, wherein N is a positive integer, and access point 102, interconnected as shown. Each station 101-i, wherein i∈{1, 2, . . . , N}, communicates wirelessly with other stations in local-area network 100 via access point 102.
Stations 101-1 through 101-N and access point 102 transmit blocks of data called frames. A frame typically comprises a data portion, referred to as a payload, and a control portion, referred to as a header. Frames transmitted from a station 101-i to access point 102 are referred to as uplink frames, and frames transmitted from access point 102 to a station 101-i are referred to as downlink frames.
Stations 101-1 through 101-N and access point 102 transmit frames over a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more of the frames can become corrupted (resulting in a collision). Consequently, local-area networks typically employ protocols for ensuring that a station or access point can gain exclusive access to the shared-communications channel for an interval of time in order to transmit one or more frames.
Such protocols can be classified into two types: contention-based protocols, and contention-free protocols. In a contention-based protocol, stations 101-1 through 101-N and access point 102 compete to gain exclusive access to the shared-communications channel, just as, for example, several children might fight to grab a telephone to make a call.
In a contention-free protocol, in contrast, a coordinator (e.g., access point 102, etc.) grants access to the shared-communications channel to one station at a time. An analogy for contention-free protocols is a parent (i.e., the coordinator) granting each of several children a limited amount of time on the telephone to talk, one at a time. One technique in which a coordinator can grant access to the shared-communications channel is polling. In protocols that employ polling, stations submit a polling request (also referred to as a reservation request) to the coordinator. The coordinator, in accordance with a polling schedule, sequentially transmits a poll to each station that specifies a transmission opportunity (TXOP) duration during which the station has exclusive access to the shared-communications channel. Since stations transmit only in response to a poll from the coordinator, polling-based protocols can provide contention-free access to the shared-communications channel.
In local-area networks where access point 102 acts as the coordinator (e.g., some Institute of Electrical and Electronics Engineers (IEEE) 802.11 networks, etc.), access point 102 combines, when possible, a payload and a poll into a single downlink frame. For the purposes of this specification, such a frame is referred to as a downlink data/poll frame.